This study was designed and approved this year, and recruitment has begun. This study has two specific aims: 1) to use longitudinal, objective techniques to characterize the clinical manifestations of early-onset bipolar llness; and 2) to identify neurophysiological correlates of affective dysregulation in bipolar children and adolescents. To accomplish Specific aim #1, we will obtain prospective clinical data (including daily mood and behavioral ratings and actigraphy data) and direct observational data, and perform neuropsychological assessments. In addition, we will assess the psychiatric status of family members, bank genetic samples from patients and first-degree relatives for use in future studies, and follow the patients longitudinally (clinically and with structural MRI scans) for 4 years. To accomplish Specific Aim #2, we will use standardized emotional stimuli to test the hypothesis that responses to negative emotional stimuli are more pronounced in children with BPD than in controls. In addition, we will test the hypotheses that, relative to controls, prepulse inhibition is reduced in children with BPD, and anterior cortical asymmetry is more variable in children with BPD.